Communication systems for communicating between locations in a building are well known. For example, intercoms are used to communicate between persons wishing to enter an apartment or office building and the occupants of the building. Typically, the person wishing to enter the building presses an electric buzzer which sounds in the room of the occupants, thereby notifying them that a person wishes to gain entrance. The occupants then press a button that electrically opens a latch or some locking mechanism of the entrance door to the building, thereby permitting the person to enter.
Another example of such communication systems is a hospital bed paging system. Typically a patient is provided with a handheld device with a call button. The patient may press the button which electrically signals the nurse or other caretaker that the patient needs assistance of some kind.
The known communication systems for communicating between locations in a building are electrically operated. However, this may pose problems in certain situations. For example, if there is a power outage and no auxiliary generator power is available, the communication system cannot operate. As another example, for religious or logistic reasons, there may be times when it is not permissible to use electricity.